The present invention relates to a radio interference detection device for detecting an interference in the same channel of radio equipment of a multi-channel access system.
In a conventional car telephone system, the multi-channel access system is adopted to effectively utilize the radio frequencies. In the multi-channel access system, a plurality of apparatuses together use a plurality of frequency channels. In the multi-channel access system of the type described above, the voice (or speech) channel of each telephone is controlled by a single common station or a single station which is connected to a plurality of local stations. According to the multi-channel access system, the control station can supervise all the mobile transmitter/receivers. If a new call is made, a channel which is not currently used can be easily assigned to this call. Therefore, in the busy mode, channel interference does not occur. Channel interference is here defined as a situation where a ratio (S/I) of the audio signal (S) to the interference output (I) is lowered by beat noise caused by an indefinite difference (e.g., about 1 kHz) between the two frequencies in the same channel, thereby degrading speech quality.
However, in a telephone system such as a cordless telephone system which has a host station (stationary station) disposed at the subscriber's terminal and a mobile station (cordless telephone set) connected to the host station through a radio relay system in one-to-one correspondence, the respective cordless telephone sets do not have a common station. Therefore, when a call is made, the corresponding cordless telephone determines a free channel to allow communication. If two or more cordless telephones using the same channel move into proximity to each other, a channel interference occurs, thereby interfering with the conversation. When the S/I ratio is degraded by beat noise, speech quality is degraded even if a ratio (D/U) of a desired wave (D) to an undesired wave (U) is high. It is very difficult to avoid the problem described above. Therefore, the multi-channel access system cannot really be applied to the cordless telephone system, although this access system allows effective use of radio frequencies.
A method is conventionally proposed in which beat noise is detected to prevent channel interference. Angle modulation is generally performed in the cordless telephone system. The ratio S/I (dB) is improved with respect to the ratio D/U (dB), as follows: EQU S/I=D/U+20 log m (1)
where m is the standard modulation factor (rad).
In general, the channel separation frequency in the cordless telephone is 25 kHz, and the standard modulation factor is 3.5 rad. An S/I ratio is improved by 11 dB (20 log m). The S/I ratio of the base-band signal is considerably greater than the ratio D/U of the radio channel signal.
S/N ratio of a noise mixed in the audio transmission channel, such as an ambient noise of the cordless telephone, i.e., S/Nr ratio does not change even after it is transmitted through a radio channel and is demodulated. In general, a standard voice of 94 dB and ambient noise of 60 dB enter the receiver of the telephone set, so that the S/Nr ratio is 34 dB. The S/N ratio of the ambient noise appeared in the demodulated base-band signal is 34 dB. In fact, radio channel noise is mixed in the base-band signal. However, in telephone equipment such as a cordless telephone set connected to the subscriber's terminal, the S/N ratio of the radio channel noise is greater than 34 dB. Therefore, it can be neglected.
Beat noise is generally detected in a manner such that zero-crossings of the base-band output are detected, digitized detection signals are counted to a predetermined value. If a certain continues for a predetermined duration, it indicates beat noise. However, this conventional detection method can be effective only when the D/U ratio is low due to the presence of ambient noise. In order to detect beat noise having the same level as that of the ambient noise, the I/Nr ratio must be about 10, i.e., 20 dB. If the I/Nr ratio is less than 20 dB, beat noise has a lower level than ambient noise. In this case, the D/U ratio is less than 3 dB in accordance with equation (1). Therefore, this conventional method is not practical since it is effective in detecting beat noise only when the D/U ratio is not more than 3 dB.